Due to increased production rates, production plants are increasingly obliged to use equipment that makes it possible to save time and reduce cycles on already highly optimized assembly procedures. One of the keys to the quality of the assemblies, particularly in terms of fatigue life, is ensuring that the assembled elements are in contact with each other, and that there is no lubricant, contamination, deformation or defect relating to piercing at the interface between the elements. Quality inspection in this area is tending to become more complex due to the spread of new practices, particularly the elimination of certain tasks, or stages, known as reference tasks, from assembly procedures.
The conventional procedures used to ensure good assembly quality are defined by the following tasks: positioning of the elements, piercing phase, separation of the elements (disassembly), deburring, cleaning, application of sealant, positioning of the elements (reassembly) and installation of the fasteners.
So-called “no-disassembly” procedures are increasingly used to ensure increased production rates. As an example, one of said procedures is defined by the following tasks: application of sealant, positioning of the elements, piercing phase and installation of the fasteners.
Although they enable a considerable time saving, “no-disassembly” procedures can generate more defects. As an example, in a conventional procedure, the presence of burrs can be inspected during the disassembly operation and rectified before reassembly, which is not possible in a no-disassembly procedure.
According to another example, contamination with cutting fluid is sometimes generated during piercing. In a conventional procedure, piercing takes place before disassembly, the interfaces are cleaned before reassembly and before application of the sealant, and the quality of the interface is therefore independent of the level of contamination generated. With a no-disassembly procedure, piercing can take place after sealant application during the sealant tack time (referring to fresh sealant); the sealant is then in a state of viscosity that can allow cutting fluid seepage at the interface.
According to a further example, it is vital that contact between the elements be ensured to avoid loss of fastener preload over time, which has an impact on the fatigue resistance of the assembly. In a conventional procedure, the bores are pinned at 100% until the intermediate sealant has cured, whereas in a no-disassembly procedure, retightening is required when the permanent fasteners are installed in fresh sealant in order to ensure satisfactory flow of the sealant for the required “metal-metal”, or at least element against element, contact.
“No-disassembly” procedures are thus generally “blind” in the sense that it is not possible to perform non-destructive inspection of the quality of the interface (in other words, the thickness of the sealant at the fasteners, the presence of swarf, burrs, cutting fluid, play, deformation or general contamination). Said new no-disassembly procedures are therefore approved on a case-by-case basis by means of ad hoc disassembly tests. Said tests cannot be generic and must be performed zone by zone, and even workshop by workshop, such is the importance of the geometry and rigidity of the elements (depending on whether small elements are being assembled or large sub-assemblies such as on the FAL (Final Assembly Line) for example) and the equipment used (piercing, pinning, jigs) with regard to the end quality. The defects or contamination present at the interfaces are then inspected visually.
Said tests require a procedure that is designed and set in advance; they are time- and energy-consuming and involve risks, for example during the handling of panels as elements (impacts, damage relating to the curing of the sealant, damage to bores/fasteners, realignment of bores, etc.). Furthermore, they cannot ensure that the quality of the assembly will endure over time, as they only give a snapshot at a given time t. The deviation of the installation conditions introduces a risk of non-conformity that becomes real if the requirements relating to the interfaces are not inspected more directly and systematically.